Add an opt-in CLI that exports each user's database to
`data/users/<user>/sqlite-backups/<YYYYMMDDTHHMMSSZ>.sqlite` (UTC) and
prunes older files to a configured count. Gated by two new settings,
`auto_sqlite_export.enabled` and `auto_sqlite_export.retention`.
Kept separate from `cli/db-backup.php` / `cli/db-restore.php`, which
stay the fixed-filename migration tool. First step of #8183.
Co-authored-by: Bjørn A. Andersen <polybjorn@users.noreply.github.com>
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